#include <bits/stdc++.h>
using namespace std;

#ifndef ONLINE_JUDGE
    #include <bits/debug.h>
#endif

#define int int64_t
#define endl "\n"

using PII = pair<int, int>;
using TII = tuple<int, int, int>;
template<class T> using V = vector<T>;
using i64 = long long;
using i128 = __int128;
using u64 = unsigned long long;

#define FOR(i, a, b) for (int i = (int)(a); i <= (int)(b); i++)
#define FOR2(i, a, b, c) for (int i = (int)(a); i <= (int)(b); i += (int)(c))
#define FORD(i, b, a) for (int i = (int)(a); i >= (int)(b); i--)
#define FORD2(i, b, a, c) for (int i = (int)(a); i <= (int)(b); i -= (int)(c))
#define ALL(a) a.begin(), a.end()
#define RALL(a) a.rbegin(), a.rend()
#define lowbit(x) ((x) & (-x))
#define RE return;
#define Yes cout << "Yes" << endl;
#define YES cout << "YES" << endl;
#define No cout << "No" << endl;
#define NO cout << "NO" << endl;
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define sz(x) (int)(x).size()

void solve() {
    int n, k, m;
    string s;
    cin >> n >> k >> m >> s;

    s = ' ' + s;

    map<char, int> mp, mp2;

    int cnt = 0, sum = 0;
    string ans;
    FOR(i, 1, m) {
        if (!mp[s[i]]) {
            mp[s[i]] = 1;
            cnt++;
            if (cnt == k) {
                sum++;
                ans += s[i];
                cnt = 0;
                mp.clear();
            }
        }
    }

    if (sum >= n) {
        YES;
    } else {
        NO;
        FOR(i, 1, n) {
            if (i - 1 < sz(ans)) {
                cout << ans[i - 1];
            } else {
                FOR(i, 'a', 'a' + k - 1) {
                    if (!mp[i]) {
                        cout << char(i);
                        break;
                    }
                }
            }
        }
        cout << endl;
    }
}

signed main() {
    int Task = 1;

    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> Task;
    while (Task--) {
        solve();
    }

    return 0;
}
